In order to keep the technology competitive for a longer period, both third generation partnership project (3GPP) and 3GPP2 are considering long term evolution (LTE), in which evolution of radio interface and network architecture are necessary.
Multimedia broadcast multicast service (MBMS) is a feature that has been introduced in the 3GPP Release 6 specifications to allow the transfer of multimedia information, (e.g., audio, video), through point-to-point (PTP) or point-to-multipoint (PTM) operation. MBMS is supported in Universal Mobile Telecommunications Systems (UMTS) release 6. In order to further improve MBMS as an efficient mechanism to reliably distribute multimedia content over evolved UMTS Terrestrial Radio Access (E-UTRA) networks to subscribers, enhanced MBMS is considered and being worked on in the 3GPP LTE standardization efforts.
In prior art of UMTS Release 6, there is no wireless transmit/receive unit (WTRU) feedback for MBMS to network base stations (called Node Bs in 3GPP). In 3GPP LTE, WTRU feedbacks has been proposed in order to improve MBMS operation in E-UTRA systems. Therefore, WTRU feedback transmission has become an important design problem in E-UTRA systems.
In MBMS, the broadcast service always operates in a PTM mode, whereas the multicast service may operate either in a PTP mode or a PTM mode. For multicast service, a radio access network (RAN) may select either the PTP mode or the PTM mode based on the number of wireless transmit/receive units (WTRUs) that have joined the multicast service within a cell using an MBMS counting procedure.
In the PTP mode, a dedicated channel (DCH) is used to carry the MBMS data to end users. In the PTM mode, data is transferred over an MBMS traffic channel (MTCH), which is mapped to a forward access channel (FACH) and in turn to a secondary common control physical channel (S-CCPCH). The PTM allows for more efficient use of radio resources when many WTRUs subscribe to the same service.
High-Speed Downlink Packet Access (HSDPA) has been introduced in 3GPP Release 5 specifications. One enhancement that has been proposed for MBMS is the transfer of MBMS data, (i.e., MTCH logical channel), over HSDPA. In HSDPA, techniques are used, such as an adaptive modulation and coding (AMC), hybrid automatic repeat request (H-ARQ) for fast feedback and fast Node B scheduling, in order to increase spectral efficiency.
The AMC adapts the transmission date rate on a high speed downlink shared channel (HS-DSCH) according to the channel conditions perceived by the receiving WTRU. The Node B may determine the best rate and scheduling for individual transmissions using the following information:
1) channel quality information (CQI) reported from the WTRUs, which indicate the quality of the channel that is perceived by the WTRUs;
2) transmit power control (TPC) commands of associated dedicated channels; and
3) acknowledgement (ACK)/non-acknowledgement (NACK) feedback for previous transmissions.
Lower data rates are generally used for transmissions to WTRUs perceiving unfavorable channel conditions, (e.g., at cell-edge). Higher data rates are used for transmissions to WTRUs perceiving favorable channel conditions.
For PTM operation, WTRUs subscribed to the multicast service receive the MBMS data over the same radio resources. In the case of MBMS over HSDPA, the WTRUs will decode the same high speed physical downlink shared channel (HS-PDSCH) codes within a given transmission time interval (TTI), (i.e., the same HS-DSCH transport block).
One challenge for the Node B is the selection of a modulation and coding scheme for transmission of the multicast transport blocks such that all WTRUs can reliably decode the data without wasting cell capacity. On one hand, the Node B may always transmit using a low data rate to ensure that all WTRUs in the cell, including those with unfavorable channel conditions, can reliably decode the data. On the other hand, the Node B may transmit at a higher data rate in order to avoid wasting HSDPA resources. In this case, WTRUs perceiving unfavorable channel conditions might have trouble decoding the multicast transport blocks.
One way of determining the best transmission rate for MBMS data is to configure WTRUs to report CQI in the uplink (UL), (i.e., CQI over a high speed dedicated physical control channel (HS-DPCCH)). The Node B may set its transmission rate according to the worst CQI report to ensure that all WTRUs can reliably decode the data. However, as the number of WTRUs subscribed to the multicast service increases, the transmission of CQI feedback information causes a significant noise rise in the uplink, reducing the capacity available for other uplink transmissions. Current 3GPP Release 6 specifications do not provide support for optimal transmission rate selection and scheduling of multicast transport blocks over HS-DSCH.